C.D. Martin

Modelling brittle failure of rock

Abstract Observations of brittle failure at the laboratory scale indicate that the brittle failure process involves the initiation, growth, and accumulation of micro-cracks. Around underground openings, observations have revealed that brittle failure is mainly a process of progressive slabbing resulting in a revised stable geometry that in many cases take the form of V-shaped notches. Continuum models with traditional failure criteria (e.g. Hoek–Brown or Mohr–Coulomb) based on the simultaneous mobilization of cohesive and frictional strength components have not been successful in predicting the extent and depth of brittle failure. This paper presents a continuum modelling approach that captures an essential component of brittle rock mass failure, that is, cohesion weakening and frictional strengthening (CWFS) as function of rock damage or plastic strain.

Quantification of rock mass damage in underground excavations from microseismic event monitoring

Abstract Rock mass damage assessment is required for many applications in rock engineering practice including support design, contamination transport control, stope design, amongst others. While various methods such as displacement measurement, seismic refraction, and direct observation using borehole camera have been used, relatively few efforts have been made to use microseismic monitoring to quantify the rock mass damage. From laboratory tests, it is well known that microseismic events are indicators of fracturing or rock damage as the rock mass is brought to failure at high stress. By capturing the microseismic events, underground excavation induced rock mass degradation or damage can be located but can the amount of damage in terms of changes to strength or deformation properties be measured? In the present study, a method of characterizing rock mass damage near excavations based on microseismic event monitoring is developed and a damage-driven numerical model is presented that takes the microseismic data as input to determine the damage state described by fracture density. The approach is built on the discovery that a realistic crack size corresponding to a seismic event can be established by applying a tensile cracking model instead of the traditional shear model, commonly used in earthquake analysis. The rock mass is softened by the introduction of cracks and this is simulated by a micro-mechanics based constitutive model. The material property input for the model are Youngs modulus, Poissons ratio of the intact rock, and information obtained from the monitoring of microseismic events such as the location and the source size of each event calculated from source parameters. Using data from the Atomic Energy of Canada Limited Mine-by Experiment, this model has been verified by investigating the linkage between microseismicity, rock mass damage and ground deformation. It is found that when damage related softening based on microseismic data is considered, predicted rock mass displacements are in good agreement with extensometer measurements.

The strength of hard-rock pillars

Abstract Observations of pillar failures in Canadian hard-rock mines indicate that the dominant mode of failure is progressive slabbing and spalling. Empirical formulas developed for the stability of hard-rock pillars suggest that the pillar strength is directly related to the pillar width-to-height ratio and that failure is seldom observed in pillars where the width-to-height ratio is greater than 2. Two-dimensional finite element analyses using conventional Hoek–Brown parameters for typical hard-rock pillars (Geological Strength Index of 40, 60 and 80) predicted rib-pillar failure envelopes that did not agree with the empirical pillar-failure envelopes. It is suggested that the conventional Hoek–Brown failure envelopes over predict the strength of hard-rock pillars because the failure process is fundamentally controlled by a cohesion-loss process in which the frictional strength component is not mobilized. Two-dimensional elastic analyses were carried out using the Hoek-Brown brittle parameters which only relies on the cohesive strength of the rock mass. The predicted pillar strength curves were generally found to be in agreement with the observed empirical failure envelopes.

Stress, instability and design of underground excavations

When the stress-induced risks to a projects warrant it, in situ stress must be measured. However, as the stress-induced risks increase, i.e., the stress magnitudes approach the rock mass strength, the confidence in commonly used stress measurement techniques decrease. The design of underground openings at depth requires knowledge of the in situ stress state, yet it is for these design conditions where our confidence in stress measurement techniques is at its lowest. To quantify the stress state for these conditions, elements of the Observational Design Method have to be used. These elements rely on the development of a geological site model, documented observations of over stressed rock in pillars or near the boundary of underground openings, and iterative two- and three-dimensional numerical modelling calibrated with observations. Examples are provided to illustrate how the philosophy of Observational Design Method can be used to infer the in situ stress state.

Measurement of in-situ stress in weak rocks at Mont Terri Rock Laboratory, Switzerland

In-situ stress measurements in weak rocks, such as clay shales, that respond strongly to environmental changes are particularly difficult. An extensive in-situ stress measurement program has been conducted at the Mont Terri Rock Laboratory in Northwestern Switzerland. Hydraulic fracturing, Undercoring around a 600-mm-diameter borehole using CSIRO triaxial strain cells and the Borehole Slotter were used to establish the state of stress for the Opalinus Clay at this location. Conflicting orientations and magnitudes resulted from the measurement programs. Three dimensional elastic modelling was used in conjunction with tunnel and borehole observations to establish the most likely stress tensor. A stress measurement program using a borehole deformation gauge is currently underway to check the stress tensor resulting from the observational modelling.

Sleeve-fracturing limitations for measuring in situ stress in an anisotropic stress environment

Abstract In order to determine the far-field in situ stress using the sleeve-fracturing technique, two and three discrete fractures must be produced for the double-fracture and single-fracture method, respectively. Numerical modelling of the sleeve-fracturing process, using the fracture mechanics code FRANC2D/L, showed that the initiation and propagation of discrete fractures, using the single or double sleeve-fracture method, was controlled by the far-field in-plane stress ratio. The numerical modelling also provided the characteristic shapes of the pressure–displacement curves for sleeve fracturing under various boundary conditions and highlighted the difficulty of detecting the inflection points in the pressure–displacement curves for the sleeve-fracture method, a key to the successful interpretation of the in situ stress data.